Dark matter is nothing mystical. Its name is also incredibly descriptive: we know that it is matter , in the sense that it has properties similar to what the matter that makes up a piece of cloth, the fuselage of a rocket, or a neutron star might have. We also know that it is dark , because it does not absorb, reflect, or emit light. It’s that simple. What we simply lack is to know what exactly it is . Evidence of its existence is not lacking.
Although the hypothesis that dark matter could exist is more than a century old, the evidence for this came more recently. As early as the late 1960s, Vera Rubin observed how various galaxies rotated , studying the speed of rotation of the stars that made up the galaxy as a function of their distance from the galactic center. What I expected was that the stars would slow down with increasing distance , in the same way that Neptune orbits slower than Mercury. However, he did not observe that, but instead obtained a speed that remained more or less constant . This meant that stars outside the galaxy would be thrown out by their high speed, unless there was a lot of mass in the galaxy that we couldn’t observe .
Similarly, the velocity of stars in a star cluster or of galaxies in a cluster of galaxies was found to be too high for the clusters studied, again suggesting the presence of more mass than was visible . Furthermore, the mass of these galaxy clusters could also be measured by other methods, such as gravitational lensing , corroborating what we already knew.
Also the cosmic microwave background , the large-scale structure of the universe or observations of type Ia supernovae , or of specific objects such as the Bullet Cluster point in that direction: there are large amounts of something with mass (and therefore with gravitational effects) but that does not interact with light and therefore we are unable to see directly. That is what we call dark matter.
The objective therefore is to find out what exactly it is made of. One possibility is that they are (at least in part) massive objects too dim to detect, such as black holes, brown dwarfs, faint old white dwarfs, neutron stars . However, direct searches for these objects have shown that their quantity must be much less than that necessary to explain dark matter and that they would not fit with what we know about nucleosynthesis after the Big Bang (the formation of different atoms). For this reason we believe that most of the dark matter must be in the form of unknown subatomic particles that do not feel the electromagnetic interaction.
These particles could take many different forms. WIMPs, axions, sterile neutrinos, supersymmetric particles , etc. We have tried to detect all of them directly and indirectly, without much success. Direct search consists of observing how a particle is ejected after colliding with a dark matter particle. For this it is necessary to eliminate the background noise to extreme levels , to make sure that if we observe something it is due to the presence of dark matter and nothing else.
The indirect search consists of observing the products of the interaction of dark matter with ordinary matter or with itself. That is, these particles could perhaps emit known particles when they interact with each other or they could annihilate into something we do understand.
Another option, of course, would be the one proposed in the title of this article: create dark matter in our laboratories . These laboratories would not be filled with test tubes and flasks, but with wires, electromagnets, and huge detectors: they would be particle accelerators . The place where it would be most likely to detect these particles today would be in the Large Hadron Collider (the LHC), since it is the accelerator where the particles reach the highest energy. However, it is most likely that even in an accelerator as powerful as this, we can only observe dark matter indirectly , because by interacting so weakly with the rest of the matter, our detectors will most likely not be able to register it.
Therefore, what we would detect is that a certain amount of energy has “disappeared” at some point. This is so because we know the energy that the original particles have, those that have collided and this energy should be conserved after the collision , no matter what quantities or types of particles are formed from it. If particles are detected, but they do not accumulate enough energy, it would mean that something has also been created whose detectors have not been able to perceive .
Currently no dark matter has been searched for at the LHC, because this collider was not designed for that purpose and efforts have been focused on other searches. However, some parameters of dark matter have been limited using data from the LEP accelerator, which instead of colliding hadrons collides leptons (specifically electrons). Any detection that these or future colliders might make should of course be checked against direct or indirect detection experiments to corroborate that the detected particles are dark matter and not anything else.
References:
Bertone, G. et al, 2005, Particle dark matter: Evidence, candidates and constraints, Physics Reports. 405, doi:10.1016/j.physrep.2004.08.031
Peter, A.H.G., 2012, Dark matter: A brief review, https://doi.org/10.48550/arXiv.1201.3942
